The manufacturing process for producing elastomeric articles from natural or synthetic rubber latex involves a curing step during which cross linking or vulcanisation occurs through sulphur groups between the polymer units.
Conventional processes for making elastomeric articles from natural or synthetic latex typically involve preparing a latex precompound, dispersion or emulsion, obtained by mixing latex, sulphur, an activator and an accelerator system, maturation for a certain period, dipping a former in the shape of the article to be manufactured into a coagulant and subsequently into the latex precompound and curing the latex present on the former.
Desirable properties of certain elastomeric articles such as tensile strength are substantially affected by the cross linking and curing stages of the manufacturing process.
The use of sulphur or sulphur containing compounds as vulcanising agent and zinc oxide as activator, the use of mixtures of inorganic salts (calcium nitrate, calcium carbonate and surface active agents) as a coagulator, and the use of vulcanising or sulphur cross linking accelerator compositions in the manufacture of rubber articles, is well known.
Conventional vulcanisation accelerators include dithiocarbamates, thiazoles, guanidines, thioureas and sulphenamides.
Certain fields, in which elastomeric articles are needed, such as the medical, health care or personal hygiene field, utilized specific types of equipment and processing techniques which enables the specific performance and regulatory requirements of the particular article produced.
The use of natural rubber latex in the manufacture of certain article such as medical gloves has been associated with disadvantageous properties such as allergic reactions, generally believed to be caused by natural proteins or allergens present within the natural rubber latex and the final product.
Synthetic elastomeric products and manufacturing processes which altogether reduce or avoid the likelihood of potential adverse reactions of the user or wearer, are of increasing interest in the medical field, particularly in the field of gloves.
A majority of glove manufacturing processes are water-based dipping systems.
It is generally known that solvent-based systems can possibly be used for poly(isoprene) and other elastomers although such solvent-based systems are poorly suited for the manufacture and molding of elastomeric articles for medical applications. One difficulty in the field of gloves for example is the design of processes and materials which will produce a thin elastomeric article having desirable properties such as high tensile strength. Another disadvantage of solvent-based systems is solvent toxicity.
Process and materials that would obviate or reduce the need for the use of toxic solvents, while at the same time yielding a product having desirable properties for medical applications, are thus still being explored.
More recently a process was proposed (WO 02/090430) for the manufacture of elastomeric poly(isoprene) articles such as medical gloves, condoms, probe covers, catheters, comprising the steps of:    1. preparing an aqueous latex composition containing an accelerator composition and a stabilizer, said accelerator composition comprising a dithiocarbamate, a thiazole and a guanidine compound;    2. dipping a former into said compounded latex composition; and    3. curing said compound latex composition on said former to form said elastomeric poly(isoprene) article.
Preferably said accelerator composition comprised zinc diethyldithiocarbamate, (ZDEC), zinc-2-mercaptobenzothiazole (ZMBT) and diphenylguanidine (DPG).
Although the use of said accelerator composition represented an improvement of the manufacturing process, it has been found that all these accelerators are capable of producing Type IV allergic response in human beings and may also possess increasingly unacceptable eco-tox and acute toxicity profiles. In addition ZDEC and ZMBT have been found to produce potentially harmful N-nitrosamines.
Another characteristic of said accelerator composition was that actually a stabilizer had to be used in conjunction with said accelerator system to prolong the stabilisation of the precured poly(isoprene) latex as is known from e.g. “Safer Accelerators for the Latex Industry”, Sakroborty K. B. and Couchman R., Latex 2004, Hamburg, 20-21 Apr. 2004, p. 75-87. Although in said publication compounding experiments with an accelerator system, comprising diisopropyl xanthogen polysulphide (DIXP) and a long alkyldithiocarbamate (ZDNC) instead of ZDEC and ZMBT but without any diphenyl-guanidine, were disclosed, the requirements for mechanical properties and cycle time (long maturation times) were not met. Although in this publication reference was made to occurrence of the required mechanical properties via film casting after maturation for 8 days at 30° C., this technique has been recognised as not suitable for the production of gloves or condoms.
Moreover, it was found in comparative compounding experiments in which DPG was omitted, that long maturation times of more then 4 weeks were necessary to obtain reasonable mechanical properties, which is unacceptable for the manufacture of gloves etc.
Recently, further work was published in “Novel Sustainable Accelerators for Latex Applications—Update”, Sakroborty K. B. and Couchman R., 8th International Latex Conference 2005, Charlotte, N. C., USA, 26-27 Jul. 2005. The authors describe a composition comprising DIXP and ZDNC without DPG to obtain desirable mechanical properties. However the method used does not involve coagulant dipping, but rather film casting, which makes the results not representative or even relevant for glove production. Moreover, the higher maturation temperatures used can be considered as not optimal for an economic industrial application.
It will be appreciated that there is still a strong need for an improved manufacturing process for gloves etc. from synthetic rubber latex, providing films from a stable precompound of latex, showing the required mechanical properties (tensile strength≧24 MPa; elongation at break≧750%) within a cycle time of 1 day, by using less amounts of vulcanising agent and activator and in particular sulphur, a safe accelerator system and a maturation time from some hours to less than 2 days.
As result of extensive research and experimentation said improved manufacturing process and acceleration system to be used therein, have surprisingly been found.